The present invention relates generally to a lighting device for an electric discharge lamp such as a mercury-vapor lamp, a sodium-vapor lamp or a metal halide lamp and, more particularly, to a current control technique during a period ranging from a starting of the electric discharge lamp to a discharge stable state thereof.
Various kinds of lighting devices for an electric discharge lamps have previously been proposed. One of these lighting devices is described in, for example, HANDBOOK OF LIGHTING, pp. 198-201, 1st edition, published on May 20, 1983, by OMU-SHA Co., Ltd., in HANDBOOK OF ELECTROTECHNOLOGY, pp. 1539-1541, 1st edition, published on Apr. 10, 1978, by DENKI-GAKKAI Corp.
Referring to FIG. 5, there is shown such known lighting device for an electric discharge lamp.
Before describing the known lighting device for an electric discharge lamp, a characteristic of change in luminous energy of the electric discharge lamp will be described. It is to be noted that the description will be made in connection with the metal halide lamp.
The metal halide lamp includes a bulb filled with a mixed gas, and a pair of electrodes which are opposite to each other across a gap.
If a high voltage is applied between the electrodes so as to induce a dielectric breakdown of the mixed gas in the bulb, an impedance of the bulb decreases from .infin.(=value when the bulb is in insulated state) to tens .OMEGA., causing a current flow between the electrodes. This current is called "discharge current".
Due to this discharge current, the temperature in the vicinity of the gap increases, and, finally, the mixed gas begins to emit a light. In response to increase in temperature in the bulb, the impedance thereof becomes higher, and the lamp assumes a negative characteristic that a terminal voltage of the bulb increases as the discharge current decreases.
Referring to FIG. 4, the luminous energy and the gas temperature are changed in a similar manner, whereas the luminous energy and the discharge current are in inverse proportion to each other, and the luminous energy and the terminal voltage are in proportion to each other. Each of the luminous energy, the gas temperature, the discharge current, and the terminal voltage undergoes a sudden change during a few seconds after starting of the lamp, and falls into a saturated state, i.e., a discharge stable state after a few subsequent seconds.
If the discharge current is so controlled as to assume a larger value (2 A, for example), a predetermined value of luminous energy is obtained quickly, however, the luminous energy of the lamp exceeds the predetermined value in the discharge stable state, resulting in decreased longevity thereof. On the other hand, if the discharge current is so controlled as to assume a smaller value (0.5 A, for example), from starting of the lamp, in response to the discharge stable state, a relatively long time is needed until the predetermined value of luminous energy is obtained.
Referring again to FIG. 5, the lighting device for an electric discharge lamp includes an AC power supply 1, a full wave rectifier 2, an inverter circuit 3, a series resonance circuit 4 having a coil L and a condenser C connected in series, and an electric discharge lamp 5.
Upon starting of the electric discharge lamp 5, if a voltage having a frequency of the order of 100 kHz, for example, is applied to the LC series resonance circuit 4 via the circuit 3, a high voltage of the order of 10 kV appears between both ends of the electric discharge lamp 5, resulting in occurrence of dielectric breakdown of a filler gas in the electric discharge lamp 5. Simultaneously with the occurrence of dielectric breakdown, if the frequency is decreased to about 10 kHz, a current of the order of 1 A passes through the electric discharge lamp 5 due to low gas temperature, i.e., low resistance value of the lamp 5. Thereafter, the resistance value of the electric discharge lamp 5 increases with a rise in gas temperature, so that the current decreases gradually. When the gas temperature becomes stable, the current comes to 0.4-0.5 A, and falls into a saturated state, thus obtaining a predetermined value of luminous energy of the electric discharge lamp 5.
When serving as a headlamp for a motor vehicle, the electric discharge lamp 5 is more advantageous than a filament lamp in view of a size, a weight, and/or an efficiency.
With the electric discharge lamp 5 as described above, however, if the voltage and frequency to be applied is so established as to obtain the predetermined value of luminous energy when the lamp 5 falls into a discharge stable state, a relatively long time is needed before the electric discharge lamp 5 comes to the predetermined value of luminous energy. On the other hand, in order to shorten the aforementioned time, if the voltage and frequency to be applied are so established as to obtain the predetermined value of luminous energy before the electric discharge lamp 5 falls in the discharge stable state, i.e., while it is in a transient state of lighting, the luminous energy of the lamp 5 exceeds the aforementioned predetermined value in the discharge stable state, resulting in decreased longevity thereof. Thus, the prior art electric discharge lamp 5 cannot satisfactorily serve as the headlamp for a motor vehicle which has to achieve the predetermined value of luminous energy immediately after starting, and must be durable in construction.